There are many different nutrients that are present in this meal. For example, the steak is a source of protein. The french fries are a source of carbohydrates. Both these nutrients help supply the body with energy.

Nutrients

Carbohydrates, proteins, and lipids contain energy. When your body digests food, it breaks down the molecules of these nutrients. This releases the energy so your body can use it.

Carbohydrates

Carbohydrates
are nutrients that include sugars, starches, and fiber. There are two types of carbohydrates: simple and complex. Pictured below are some foods that are good sources of carbohydrates (
Figure
below
).

Up to the age of 13 years, you need about 130 grams of carbohydrates a day. Most of the carbohydrates should be complex. They are broken down by the body more slowly than simple carbohydrates. Therefore, they provide energy longer and more steadily.

Simple Carbohydrates

Sugars are small, simple carbohydrates that are found in foods such as fruits and milk. The sugar found in fruits is called fructose. The sugar found in milk is called lactose. These sugars are broken down by the body to form glucose (C
6
H
12
O
6
), the simplest sugar of all.

Where does glucose come from? Recall that glucose is the product of photosynthesis, so some organisms such as plants are able to make their own glucose. As animals cannot photosynthesize, they must eat to obtain carbohydrates. Through the process of
cellular respiration
, glucose is converted by cells into energy that is usable by the cell (ATP).

Complex Carbohydrates

Starch
is a large, complex carbohydrate made of thousands of glucose units (monomers) joined together. Starches are found in foods such as vegetables and grains. Starches are broken down by the body into sugars that provide energy.

Fiber
is another type of large, complex carbohydrate that is partly indigestible. Unlike sugars and starches, fiber does not provide energy. However, it has other important roles in the body. For example, fiber is important for maintaining the health of your gastrointestinal tract. Eating foods high in fiber also helps fill you up without providing too many calories. Most fruits and vegetables are high in fiber. Some examples are pictured below (
Figure
below
).

Between the ages of 9 and 13 years, girls need about 26 grams of fiber per day, and boys need about 31 grams of fiber per day.

Proteins
are nutrients made up of smaller molecules called
amino acids
. Recall that there are 20 different amino acids arranged like "beads on a string" to form proteins. These amino acid chains then fold up into a three-dimensional molecule, giving the protein a specific function. Proteins have several important roles in the body. For example, proteins make up antibodies, muscle fibers and enzymes that help control cell and body processes. You need to make sure you have enough protein in your diet to obtain the necessary amino acids to make your proteins.

If you eat more than you need for these purposes, the extra protein is used for energy. The image below shows how many grams of protein you need each day (
Figure
below
). It also shows some foods that are good sources of protein.

Between the ages of 9 and 13 years, you need about 34 grams of proteins a day. Seafood and eggs are other good sources of protein.

Lipids
are nutrients, such as fats that store energy. Lipids also have several other roles in the body. For example, lipids protect nerves and make up the membranes that surround cells.

Fats are one type of lipid. Stored fat gives your body energy to use for later. It’s like having money in a savings account: it’s there in case you need it. Stored fat also cushions and protects internal organs. In addition, it insulates the body. It helps keep you warm in cold weather.

There are two main types of fats, saturated and unsaturated.

Saturated fats
can be unhealthy, even in very small amounts. They are found mainly in animal foods, such as meats, whole milk, and eggs. So even though these foods are good sources of proteins, they should be eaten in limited amounts. Saturated lipids increase cholesterol levels in the blood. Too much cholesterol in the blood can lead to heart disease.

Unsaturated fats
are found mainly in plant foods, such as vegetable oil, olive oil, and nuts. Unsaturated lipids are also found in fish, such as salmon. Unsaturated lipids are needed in small amounts for good health. Most lipids in your diet should be unsaturated.

Another type of lipid is called
trans fat
. Trans fats are manufactured and added to certain foods to keep them fresher for longer. Foods that contain trans fats include cakes, cookies, fried foods, and margarine. Eating foods that contain trans fats increases the risk of heart disease.

Beginning with Denmark in 2003, many nations now limit the amount of trans fat that can be in food products or ban these products all together. On January 1, 2008, Calgary became the first city in Canada to ban trans fats from restaurants and fast food chains. Beginning in 2010, California banned trans fats from restaurant products, and in 2011, from all retail baked goods.

Vocabulary

amino acid
: Small molecule used to build proteins.

carbohydrate
: Organic compound such as sugar and starch that provides an energy source for animals.

cellular respiration
: Process of breaking down glucose to obtain energy in the form of ATP.

fiber
: Carbohydrate that is partially indigestible.

lipid
: Organic compound that is insoluble in water and includes fats, oils, and waxes.